centering fluid background image? css/jquery

Sorry if this is super basic (first time in the forums, setting up a site for a nonprofit) but I’ve tried a bunch of different plugins, searched for a while, and can’t quite get any to do what I want with the theme I purchased.

What I’m after is simply this: add one background image to the entire site that is vertically aligned to the top and horizontally centered, regardless of browser size. I do not want the image to resize, any horizontal excess can just be hidden.

The theme I’m using is Accende, by wpaddicts, which had the built in wordpress background support disabled. I tried enabling that, but the image didn’t show up (some other code over-riding?) so I started trying various plugins. Here’s the plugin/code I’ve been messing with:

<?php
/**
 * @package Full screen background
 * @version 0.6
 */
/*
Plugin Name: Full screen background
Plugin URI: http://www.trappers.tk/share/fullbg.zip
Description: Full screen background, like the google home page
Author: Jeroen Trappers
Version: 0.5
Author URI: http://blog.trappers.tk
*/

function wp_full_bg() {
?>
   <!-- Change the src attribute of the img tag just below here to point to the image you want to use. -->
   <img id="wpfullbg" src="http://www.peterandjohn.org/wp-content/uploads/2013/01/site-wallpaper.jpg" >

   <script>window.jQuery || document.write('<script src="//ajax.googleapis.com/ajax/libs/jquery/1.5.1/jquery.min.js"><\x3C/script>')</script>
   <script type="text/javascript" language="javascript">
     jQuery(function() {
     	fullbg();
	// jQuery(window).resize(fullbg);
	setTimeout(fullbg, 500);
     });
     function fullbg() {
       var imgw = jQuery('#wpfullbg').width();
       var imgh = jQuery('#wpfullbg').height();
       var vpw = jQuery(window).width();
       var vph = jQuery(window).height(); 

       var imgratio = imgw / imgh;
       var viewportratio = vpw / vph;

       var w = vpw;
       var h = vph;

       if( viewportratio > imgratio )
       {
         // screen is wider than picture -> increase the height of the picture;
         h = w / imgratio;
       }
       else
       {
         w = h * imgratio;
       }

       jQuery('#wpfullbg').css(
          {
             // 'left': '0pt',
             'align': 'center',
             'width': 1903,
             'height': 995,
             'overflow': 'hidden',
             'z-index': '-2',
             'top' : '0px',
             'position' : 'fixed',
             'opacity' : '0.999999999'
          }
       );
     }
   </script>
<?php
}
add_action( 'wp_footer', 'wp_full_bg' );
?>

Any help is greatly appreciated, thank you! Website is peterandjohn.org